Neurotransmitter receptors provide access to neuronal circuits for investigation of their biology, and potentially for therapeutic intervention. Peptide effects on G-protein coupled receptors are often relatively long lasting and may have a modulatory role. These subtle effects may be relevant to the basis or treatment of mental illness. We are studying a family of G-protein coupled receptors (referred to as the secretin-VIP family, family B, or type II) that is structurally distinct from the majority of G-protein coupled receptors (rhodopsin- or beta-adrenergic-like receptors). Recently we have focussed on the parathyroid hormone 2 (PTH2) receptor, which we identified in a screen for novel central nervous system expressed polypeptide receptors. Our initial studies of the human PTH2 receptor showed that it was activated by parathyroid hormone (PTH). Mapping the PTH2 receptor?s cellular distribution revealed expression within brain areas not normally exposed to circulating peptides, such as PTH. Further, we found that PTH poorly activated the rat PTH2 receptor. Using selective activation of the PTH2 receptor as an assay we purified a previously unknown 39 amino acid peptide from bovine hypothalamus. This peptide, which we named tuberoinfundibular peptide of 39 residues (TIP39), activates PTH2 receptors from human, rat and zebrafish and does not activate PTH1 receptors. Both rat and zebrafish PTH2 receptors are much more effectively activated by TIP39 than PTH, supporting the suggestion that a TIP39 homologue is their natural ligand. We are currently testing hypotheses for the biological role of TIP39 and the PTH2 receptor derived from recent detailed mapping of the PTH2 receptor?s distribution. We have determined the sequence of rodent and human TIP39 and are currently mapping the peptide?s distribution. We are attempting to develop mice with PTH2 receptor and TIP39 null phenotypes to aid in elucidating the function of this peptide-receptor system.